1. Field of the Invention
This invention relates to microcircuit packaging in general, and in particular, to a method and apparatus for protecting hypersensitive microcircuits on a semiconductor wafer from contamination and mechanical damage during die sawing and subsequent die-handling operations.
2. Description of the Related Art
Microcircuits are typically fabricated on the surface of a wafer of a semiconductor material, e.g., silicon, in a rectangular array of identical devices. The typical manufacturing process involves numerous manufacturing steps, such as cleaning, printing, etching, doping, coating, plating, and ion implantation. Upon completion of these steps, and prior to the packaging of the individual microcircuits for use in electronic devices, the microcircuits, together with an underlying portion of the wafer, are separated from wafer into individual dies, or xe2x80x9cchips.xe2x80x9d This separation is typically accomplished by a mechanical sawing operation, or by breaking the wafer along scribe lines created in the wafer by a laser or a diamond-tipped scribe.
In conventional microcircuit die sawing practice, a layer of a sticky tape, such as that sold by the Nitto Denko Corporation of Osaka, Japan under the name xe2x80x9cNitto tape,xe2x80x9d or that sold by the Lintec Company of Shiga, Japan, is attached to the backside of the wafer, which is then placed face up on the saw table, and sawn through down to, but not including, the tape backing. The individual dies are then removed from the tape by automated xe2x80x9cpick-and-placexe2x80x9d die-handling equipment that includes a needle that pierces the sticky tape from the underside to contact the bottom surface of the die and separate it from the tape, and an arm that grasps the upper, circuit-containing surface of the die with, e.g., a vacuum collect, and transports it to another location for subsequent processing
Typical microcircuits manufactured in the above manner are substantially flat, i.e., the circuit components and elements are closely integrated with each other, are substantially planar in form, and are typically on the order of a few angstroms to a few microns thick. As such, they are moderately resistant to contamination by dust particles and vapors generated during the die-sawing operation, as well as to mechanical damage occasioned by handling of the dies during die sawing and subsequent manufacturing operations. In some instances, this resistance to contamination and/or mechanical damage can be enhanced by the vapor deposition of a protective layer of silicon dioxide or silicon nitride on the face of the microcircuits prior to die separation.
However, there are at least two classes of microcircuit devices that are highly sensitive to contamination by die-sawing byproducts and/or to mechanical damage occasioned by handling during manufacturing, namely, the so-called xe2x80x9coptical sensorxe2x80x9d and xe2x80x9cmicro-machinexe2x80x9d devices. An example of the former would include the xe2x80x9cmicro-mechanical display logic and arrayxe2x80x9d of A. M. Hartstein, et al., (U.S. Pat. No. 4,229,732), while examples of the latter would include the xe2x80x9celectrostatic motorxe2x80x9d described by R.T. Howe, et al. (U.S. Pat. No. 4,943,750), or the xe2x80x9cmachine structuresxe2x80x9d made by the method of Sparks et al. (U.S. Pat. No. 5,427,975).
These latter types of devices have in common that both types include highly fragile micro-structures and/or specialized reflective surfaces that either extend, or face, upward from the face of the die, and they may also include microscopic openings into the underlying semiconductor substrate, such as might be found in an integrated circuit pressure transducer. For obvious reasons, these structures are highly susceptible to both contamination by the dust, cooling liquids, and/or vaporous by-products generated by die-sawing, as well as to the mechanical damage that could result from, e.g., a slight, unintended gust of air or drop of water incident on the face of the wafer. Such contamination or damage could result in an entire wafer of relatively expensive devices being ruined and scrapped.
Accordingly, special manufacturing procedures and equipment are needed to handle these hypersensitive types of devices. This is particularly so at the stage of their manufacture at which the micro-features are fully defined on the face of the wafer or on the separated dies, such as during the die-sawing operation, or during subsequent die-mounting procedures. The prior art methods and apparatus for dealing with these special types of microcircuits described below, while workable, have some associated drawbacks that adversely effect their efficiency.
The prior art method for die-sawing these hypersensitive types of microcircuits is described in some detail in U.S. Pat. No. 5,362,681 to C. M. Roberts, Jr., et al. The method includes inverting the wafer face down on the saw table and sawing it from the back face of the wafer. To protect the microcircuit devices on the front face of the wafer, the wafer is attached to a spacer film, typically a Mylar tape, carried on a stretcher frame. The film has a pattern of openings in it corresponding to the array of dies on the face of the wafer, and is adhered to the front face of the wafer, rather than to its backside, as is done with conventional microcircuits. The spacer film is sized such that its periphery overhangs the margin of the wafer. Four sets of alignment holes, oriented with respect to the xe2x80x9cstreetsxe2x80x9d between the dies, are punched into the tape on opposite sides of the wafer outside of its margin. A second film is then adhered to the backside of the spacer film to seal the component openings in the spacer film.
The wafer is then placed upside-down on the saw table, and aligned with respect to the saw blade by means of an alignment system that aligns the wafer with respect to the above-described four sets of alignment holes in the spacer film. The wafer is sawed through its back side down to, but not through, the spacer film to singularize the dies from the wafer. The dies are then individually pushed and lifted from the spacer film by means of specially designed pick-and-place equipment that includes a special, hollow xe2x80x9cneedle clusterxe2x80x9d that pushes upwardly through the spacer film to contact the edges of the die to separate it from the film, and an arm that grasps the die from the back side with a vacuum collet. The arm then inverts the die 180 degrees such that its front face faces upward, then hands the die off to a second arm also equipped with a special hollow vacuum collet that enables the arm to grasp the sensitive front side of the die without damaging the microcircuit thereon.
While the above prior art method is workable, it has several drawbacks associated with it: First, since the wafer is sawn upside down, the underside of the dies, rather than their top surfaces, are presented for removal of the dies from the spacer film. This prevents the use of conventional automated pick-and-place equipment, and necessitates the use of the specially adapted pick-and-place apparatus described above to accommodate the sensitive micro-structures located on the top side of the die. It would be desirable if conventional pick-and-place die handling equipment could be used with these hypersensitive types of chips.
Also, because removing the dies from the spacer film destroys the sealed enclosure that protects the microcircuits during the sawing operation, once the dies are removed from the spacer film, they must thereafter be maintained in a clean room environment and are at increased risk of mechanical damage and/or contamination until they have been individually packaged in a protective enclosure. It would therefore be desirable if the protection afforded the delicate microstructures during the sawing operation could be retained with the individual dies after sawing so that they could be safely handled and stored in a less critical environment.
Further, because the wafer is sawn face down on the table, the scribe lines on its face, and indeed, the microcircuits themselves, are not directly accessible for saw alignment purposes. Instead, the saw must be indirectly aligned with the wafer by means of the four sets of alignment holes in the film spacer described above. Since there is a tolerance buildup between the various parts, this results in a loss of precision in the location of the saw cuts, and thus necessitates a wider spacing, or street, between devices to accommodate the tolerance buildup. In particular, the width of the scribe lines necessitated by this prior art method is 12 mils, i.e., four times as great as the conventional die spacing, or scribe line width, possible with a wafer that is sawn face up. This translates into a waste of wafer space and a concomitant reduction in device-per-wafer yield.
What is needed, then, is an effective method and apparatus for handling these types of fragile and contamination-sensitive microcircuits that overcomes the above problems in a simple, inexpensive, and efficient manner.
According to this invention, a simple, inexpensive, and efficient method and resulting structure are provided for protecting the microcircuits on the face of a semiconductor wafer from contamination and damage during wafer sawing and subsequent die attachment procedures. The method comprises forming a sealed, protective dome over each of the microcircuits on the face of the wafer such that, when the individual microcircuits and associated dies are sawn from the wafer, the protective dome over each of the microcircuits remains associated with and protectively sealed over its associated microcircuit.
In one embodiment of a saw protector, a first sheet of material is selected, preferably plastic, having an area sufficient to cover all of the microcircuits in the wafer. A pattern, or array, of protective domes corresponding to the array of microcircuits on the wafer is formed into the sheet, e.g., by molding or thermo forming. Each of the domes is formed to have a height greater than the maximum height of any etched or mechanical feature extending upward from the microcircuits, and a periphery at least as great as the periphery of its corresponding microcircuit.
The edges of the sheet are preferably trimmed such that, when the sheet overlays the wafer, the ends of conventional saw-alignment scribe lines on the face of the wafer are exposed for use by either manual or automated optical saw alignment equipment. The sheet is oriented with the dome openings facing toward the wafer, and is aligned with respect to it such that each of the domes is disposed over a corresponding microcircuit. The sheet is then impermanently adhered to the face of the wafer, preferably by means of a pressure-sensitive adhesive having a peel-off protective backing previously applied to the sheet, such that each microcircuit on the face of the wafer is individually covered by and protectively sealed within its own corresponding dome during die-sawing and subsequent die handling operations.
When the dies are thereafter separated from the wafer by sawing of the wafer, the wafer is sawn with its front side facing up, and the cut is made to pass along the scribe lines between the dome-covered dies, and to cut through both the domed sheet and the wafer so that a portion of the sheet, including the protective dome over each microcircuit, remains attached to and protectively sealed over its corresponding microcircuit and associated die.
In an alternative embodiment, the sheet is selected from a material having a thickness greater than the maximum die-feature height, and the domes are partially defined, e.g., by die-cutting, to comprise openings that extend through the entire thickness of the sheet. These through-openings facilitate alignment of the sheet with the wafer, as the desired disposition of the microcircuits within the openings may be readily visualized through the upper surface of the sheet during its alignment with the wafer using automated optical pattern recognition methods. A second sheet having a periphery conforming to the first sheet is then adhered to the top surface of the first sheet to close off the through-openings and thereby form individual, sealed protective domes over each microcircuit. As with the first embodiment, the saw cut is made from the front face, or top surface of the wafer, with the saw passing along the scribe lines between the dies and through both sheets of the saw protector, and then through the thickness of the wafer such that, when the dies are separated from the wafer, each is accompanied by its own sealed protective dome over the associated microcircuit. The die can be picked up and handled by means of the domed enclosure, which accompanies the die and protects the microcircuit on it, both during the sawing operation and subsequently, e.g., during die attachment, yet the enclosure is easily removed at a later stage by simply peeling it away from the chip.